Such droplet spray devices as a battery-operated liquid Droplet spray device are also sometimes called aerosol generators, nebulizers and the like. They normally contain a nozzle body on a support part, in particular, a nozzle body of a liquid droplet spray device which dispenses a liquid substance as a liquid droplet spray or from the device through the nozzles of the nozzle body. They further consist of a piezoelectric actuator used as a vibrating element for causing the liquid to vibrate so to be accelerated and expelled as droplets. They further consist of elements such as a liquid space, liquid feed and fluid interface to a reservoir, a reservoir as well as electrical connections between the vibrating element and a corresponding electronic circuitry. The liquid may be for example an ambient fragrance, a perfume, an insecticide, an aromatherapy essence, a liquid pharmaceutical formulation, aqueous based liquids and flammable or combustible liquids.
Such nozzle bodies are sometimes called aperture plates, nozzle arrays, dosing aperture, orifice plate, vibratable membrane member, dosing aperture arrangement, aerosol generator and the like. Such terms are hence to be understood as being interchangeable throughout the present document.
In fact such nozzle bodies and droplet spray devices are well known. For example see the document EP 1 129 741 in the name of the present Applicant. This document describes a liquid droplet spray device having a top substrate formed of a main body and of a nozzle body. The nozzle body contains a nozzle array of liquid droplet outlet means allowing a liquid substance contained in the liquid droplet spray device to exit the device, in this case as a spray of droplets. A piezoelectric actuator is used to cause the liquid to undergo a vibration so as to generate the droplet spray.
Generally, such piezoelectric actuator is driven so as to oscillate at or near its resonance frequency to improve energy efficiency.
Further, it is known that temperature deviations result in changes of the electrical characteristics so that it is important to control the operating frequency of the piezoelectric actuator.
Also, once a reservoir is empty, it is preferable to stop the piezoelectric actuator so as to avoid wasting energy. Moreover, if the piezoelectric actuator continues functioning once the reservoir is empty, there is a risk that the liquid supply means will dry out. When a wick is used for supplying the liquid this has a disadvantage that undesirable smells may emanate from the wick in such circumstances. Once a new reservoir is added, it may be difficult to control the amount of liquid that will be expelled, because the wick must first be saturated with liquid before it can function properly, in particular with a heat-diffusing device.
Many piezoelectric actuator control systems are known as such.
For example, the article “Vibration control of flexible beams using self-sensing actuators” by F. Pourboghrat et al., Electr. & Comput. Eng., Southern Illinois Univ., Edwardsville, Ill., USA; World Automation Congress, 2002. Proceedings of the 5th Biannual, Volume: 14, pages 133-139, ISBN:1-889335-18-5 describes a design and implementation of a smart piezoelectric actuator for active vibration control of flexible structures. A disturbance estimation technique is used for estimating the induced strain on a piezoelectric actuator, which results in a self-sensing piezo-actuator. The proposed self-sensing piezo-actuator is then applied for the elimination of vibration in a flexible beam structure, using a simple rate-feedback control. Simulation and experimental results for a cantilever beam are shown to demonstrate the accuracy of the proposed self-sensing actuator for vibration control.
The document EP 1 043 162 describes an inkjet apparatus having a liquid detection method using an infrared detector to determine if liquid has passed through a spray path or not. Control means are provided to adjust the spraying itself.
Document EP 1 169 568 describes a fuel injector using a piezoelectric actuator that acts on the fuel injector when voltage is applied to it. The resulting mechanical load applied to the piezoelectric actuator will generate an electric charge such that the electric charge is proportional to the mechanical load. The electric charge will be retained as long as the mechanical load is applied. By monitoring the voltage of the electric charge, it is possible to control the injected fuel. However, the fuel injector described in this document only relates to the use of one liquid substance, i.e. fuel, and does not at all detect the status (empty or not) of the liquid reservoir.
The document DE 10 2006 002 736 describes another example of a fuel injection. By measuring the current supplied to the piezoelectric actuator, it is possible to control the operation thereof. This document is also silent about the state of the reservoir, and only uses one type of liquid.
A further example of such system is described in the document DE 10 2006 004 765.
The document US 2007/0216256 describes a drive control circuit for a piezoelectric activated pump. By measuring the internal impedance of the piezoelectric actuator, it is possible to control the operation frequency.
Document US 2003/0146300 describes a nebulizer for nebulizing a substance and a reservoir having a metering chamber arranged so as to feed a substance to be nebulized to the nebulization device and a second chamber arranged to hold and retain any of this substance in excess of the volume held in the metering chamber. The device allows detecting the ejection of a unit dose, so that in principle an empty reservoir may be detected, but this is done by measuring the amount of ejected substance by way of the metering chamber.
The document U.S. Pat. No. 6,546,927 describes an aerosol generator having frequency control means that uses the internal impedance of the piezoelectric actuator to track the actual frequency. By measuring the current, the frequency applied can be controlled, seeing that at resonance, the current is lowest, as the internal impedance is lowest. However, this point is not the most stable, i.e. a slight shift in frequency can cause a large shift in impedance change as shown in FIG. 2 of this document, so that a trade-off in consumed current and frequency needs to be made.
However, this document, or any of the other cited documents, is silent about any influence or control of liquid viscosity or of ambient temperature which both have a direct impact on the operation characteristics of a piezoelectric actuator. For example, in the above-cited U.S. Pat. No. 6,546,927 a change in the ambient temperature may cause a shift of the frequency away from the resonance frequency.
It is, therefore, an object of the present invention to provide an innovative piezoelectric actuator control system for a liquid droplet spray device that overcomes the inconveniences and limitations presented by the prior art documents.